Noise sources can be categorised as an active and passive.

Active sources such as optical plugs,lasers, receivers, and amplifiers generate noise in the fiber link.

Passive sources such as connectors, fiber, splices, and WDMs cause interference by distorting or reflecting the propagating signal power.

Below are ten major noise sources:

1. Signal-spontaneous noise (si-sp): This type of noise is generated by the signal mixing with amplified spontaneous emission noise generated in an optical amplifier. It is typically the dominant noise source in an amplified optical link. It is also known as amplified spontaneous emissions (ASE) noise.

2. Spontaneous-spontaneous noise (sp-sp): This type of noise is generated by ASE mixing with itself.

3. Shot noise (sh): This kind of electrical noise is generated by the receiver photodiode in both PIN and APD type receivers. APD type receivers have a better signal to noise ratio (SNR) due to their internal multiplication gain mechanism. Receiver noise level and minimum OSNR are established in transceiver design and cannot be controlled by system planning other than by selecting better transceivers.

4. Shot-spontaneous noise (sh-sp): This kind of noise is generated by shot noise added along-with ASE noise in the receiver. It is accounted for the transceiver’s OSNR specifications.

5. Thermal noise (th): This type of noise is generated by the front end of the receiver diode due to thermal activity and is accounted for in the transceiver’s OSNR specifications. The RMS photodiode

6. Multiple path interference noise (MPI): This noise is generated by the signal reflecting multiple times in the fiber and interfering with itself. Reflections are due to Rayleigh scattering and other reflective events such as connectors and splices. It is typically a concern for high signal powers and in distributed Raman amplifiers with high gains. This noise source is also known as double Rayleigh scattering (DRS). To keep MPI noise to a minimum, use good-quality, and clean ultra-polish (UPC) or angle polish (AP) type fiber connectors. Fiber splices that show OTDR reflections should be re-spliced.